One conventional method proposed to record a halftone image by using image reproducing scanner devices such as those applied to photochemical processesis providing a lith-type film as a photosensitive material, placing a contact screen on the film, and exposing the film through a contact screen to a beam modulated by a data signal of an original picture. Although this is a simple method, it has a number of disadvantages. For example, it is preferred to place the contact screen on the lith-type film, and the contact screen is difficult to handle. Also, the halftone dots are produced by taking advantage of the high gamma characteristic of the lith film, resulting in an unstable size of the resultant halftone dots owing to the influence of fringing. This is a troublesome procedure, and sometimes results in redoing or retouching, such as dot-etching. Furthermore, because high intensity exposing light is needed, the cost of the equipment is high.
Another method for recording a halftone image is to record the halftone image with a beam through a lens and a contact screen which are set apart from a lith film and to keep a synchronous scanning relation with the lith film. This method, however, needs an additional drum for the contact screen along with the drum for an original picture and the lith film. The result is a large and complex system, with problems such as instability of dot size and light intensity remaining.
In a further method, a synthesized electric signal is obtained from individually and synchronously controlled scanning of an original picture and a contact screen to record a halftone image. This method, however, also requires an additional drum resulting in a large and complex system.
In contrast to these photographic methods, there is also, in the prior art, an electronic method to record a halftone image. This method involves dividing a single dot unit area of a contact screen which has a certain characteristic of gradation reproducibility, storing density distribution data of the divided areas into a memory and synthesizing these data with a scanned signal of an original picture. That method, however, is inferior in gradation reproducibility and has low resolving power and discontinuous gradation of density, when the division is too coarse. On the other hand, when fine division is performed, said defects are reduced, but other defects exist such as the requirement of high equipment cost owing to large memory devices. Also, the longer scanning time required in this method leads to lower manufacturing performance.
In another method to produce a halftone plate, disclosed in my Japanese Patent Publications No. 54-21123 and No. 55-44940, halftone dots are obtained on a photosensitive material with a flat exposing beam driven by an acoustic optic modulator (AOM) which controls width and position of the flat beam in accordance with the data signal corresponding to an original picture and positions of the halftone dots. This type of electrical method of recording a halftone image, however, usually requires more complex equipment.